[13][19][20] On 26 November 2021, the WHO's Technical Advisory Group on SARS-CoV-2 Virus Evolution declared PANGO lineage B.1.1.529 a variant of concern and designated it with the Greek letter omicron.

[22] The name of the variant has occasionally been mistaken as "Omnicron" among some English speakers, due to a lack of familiarity with the Greek alphabet, and the relative frequency of the Latin prefix "omni" in other common speech.

[35] Indeed, in order to be affected by such a high number of mutations, the virus must have been able to evolve a long time without killing its host, which can occur in people with a weakened immune system who receive enough medical care to survive.

[33] On 1 December 2022, a team of researchers from the Charité (Berlin) published a now-retracted study in Science that claimed that "data revealed genetically diverse Omicron ancestors already existed across Africa by August 2021".

The person arrived in Darwin on a repatriation flight from Johannesburg, South Africa on 25 November, and was taken to a quarantine facility, where the positive test was recorded.

[77] The same day, the Norwegian Institute of Public Health confirmed that 50 attendees of a company Christmas party held at a restaurant in Norway's capital, Oslo, were infected with the Omicron variant.

[95] On the same day, Novavax stated that it was developing an updated vaccine requiring two doses for the Omicron variant, which the company expected to be ready for testing and manufacturing within a few weeks.

[102] On 26 November 2021, the WHO asked nations to enhance surveillance and sequencing efforts, submit complete genome sequences and associated metadata to a publicly available database, such as GISAID, report initial cases/clusters associated with virus-of-concern infection to the WHO through the International Health Regulations (IHR) mechanism, where capacity exists and in coordination with the international community, perform field investigations and laboratory assessments to improve understanding of the potential impacts of the virus of concern on COVID-19 epidemiology, severity, and the effectiveness of public health and social measures, diagnostic methods, immune responses, antibody neutralization, or other relevant characteristics.

[105] As with other variants, the WHO recommended that people continue to keep enclosed spaces well ventilated, avoid crowding and close contact, wear well-fitting masks, clean hands frequently, and get vaccinated.

Other countries that also implemented travel bans include Japan, Canada, the European Union, Israel, Australia, the United Kingdom, Singapore, Malaysia, Indonesia, Morocco, and New Zealand.

[113] On 27 November 2021, Switzerland introduced obligatory tests and quarantine for all visitors arriving from countries where the variant was detected, which originally included Belgium and Israel.

[117] On 29 November 2021, the WHO warned countries that the variant poses a very high global risk with severe consequences and that they should prepare by accelerating vaccination of high-priority groups and strengthening health systems.

WHO director-general Tedros Adhanom described the global situation as dangerous and precarious and called for a new agreement on the handling of pandemics, as the current system disincentivizes countries from alerting others to threats that will inevitably land on their shores.

"[118] In December 2021, multiple Canadian provinces reinstated restrictions on gatherings and events such as sports tournaments, and tightened enforcement of proof of vaccination orders.

British Columbia expressly prohibited any non-seated "organized New Year's Eve event",[119][120][121] while Quebec announced a partial lockdown on 20 December 2021, ordering the closure of all bars, casinos, gyms, schools, and theatres, as well as imposing restrictions on the capacity and operating hours of restaurants, and the prohibition of spectators at professional sporting events.

[174] On 20 October 2022, the chief scientist of the World Health Organization (WHO), Soumya Swaminathan, warned that the XBB subvariant of Omicron may cause infections in some countries while the severity of the new variants is not yet known.

[197] Early indications from data collected in South Africa suggested BA.4 and BA.5 have a significant growth advantage over BA.2, which by 12 May earned the status Variant of Concern by the European Centre for Disease Prevention and Control, and, by 20 May, by the UK Health Security Agency.

[226] By July 2024, a descendant of KP.2 with an extra amino acid change in the spike protein, Q493E, was given the names KP.3 and, informally, 'FLuQE,' and became a major variant in New South Wales during the Australian winter.

[227] In January 2022, William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center, compared the contagiousness of the Omicron variant to that of the measles.

[232] A preprint supports such an explanation of Omicron's spread, suggesting that it "primarily can be ascribed to the immune evasiveness rather than an inherent increase in the basic transmissibility".

Many cases of infection broke through, albeit "a bit less than in the control group", even though trial participants had higher antibody levels after the fourth dose.

"[259] Professor Francois Balloux of the Genetics Institute at University College London said, "From what we have learned so far, we can be fairly confident that – compared with other variants – Omicron tends to be better able to reinfect people who have been previously infected and received some protection against COVID-19.

[279][280] A study performed between 1 and 7 December 2021 by the Center for Disease Control found that: "The most commonly reported symptoms [were] cough, fatigue, and congestion or runny nose" making it difficult to distinguish from a less damaging variant or another virus.

[281] Research published in London on 25 December 2021 suggested the most frequent symptoms stated by users of the Zoe Covid app were "a runny nose, headaches, fatigue, sneezing and sore throats.

[296] As of December 2021, Denmark and Norway have regarded cases found by their variant qPCR test, which is relatively fast and checks several genes,[297] as sufficient for counting it as Omicron, before full sequencing.

[308] Similar results are confirmed by cryo-electron microscopy and X-ray data, also providing the structural approach and molecular basis for the evasion of humoral immunity exhibited by Omicron antigenic shift as well as the importance of targeting conserved epitopes for vaccine and therapeutics design.

[309] As of December 2021, most monoclonal antibodies had lost in vitro neutralizing activity against Omicron, with only 3 out of 29 mAbs examined in another study retaining unaltered potency.

[311] February 2022 data suggested Omicron caused significant humoral immune evasion, while neutralizing antibodies targeting the sarbecovirus conserved region remained most effective.

[322] The National Institute for Communicable Diseases reports that children under the age of 2 make up 10% of total hospital admissions in the Omicron point of discovery Tshwane in South Africa.

[323] Data on the S gene target failure (SGTF) of sampled cases in South Africa indicates a growth of 21% per day relative to Delta, generating an increased reproduction number by a factor of 2.4.